unions

Whenever I see examples of union, they are always different types. For example, from MSDN: // declaring_a_union.cpp union DATATYPE // Declare union type { char ch; int i; long l; float f; double d; } var1; // Optional declaration of union variable int main() { } What happens if I have a union (in this case anonymous, but that shouldn't matter) like this: union { float m_1stVar; float m_1stVarAlternateName; }; Regardless of whether this is good practice or not, will this cause any issues? No, this won't cause any issues. The reason you don't see it more often is that it's pointless - both names

When I print the size of a union like this: union u { char c[5]; int i; } un; using this: int _tmain(int argc, _TCHAR* argv[]) { printf("size of union = %d ",sizeof(un)); return 0; } I get an answer of 8 using Visual C++, but I expected 5. Why? Well, for the same example, i did something like this: int i1 = 0x98761234; un.i = i1; printf("\n un.c[0] = %x ",un.c[0]); printf("\n un.c[1] = %x ",un.c[1]); printf("\n un.c[2]= %x ",un.c[2]); printf("\n un.c[3] = %x ",un.c[3]); printf("\n un.c[4] = %x ",un.c[4]); printf("size of union = %d ",sizeof(un)); i got results like un.c[0] = 34; un.c[1] = 12;

Sort of related to my previous question : Do elements of arrays count as a common initial sequence? struct arr4 { int arr[4]; }; struct arr2 { int arr[2]; }; union U { arr4 _arr4; arr2 _arr2; }; U u; u._arr4.arr[0] = 0; //write to active u._arr2.arr[0]; //read from inactive According to this cppreference page : In a standard-layout union with an active member of non-union class type T1, it is permitted to read a non-static data member m of another union member of non-union class type T2 provided m is part of the common initial sequence of T1 and T2.... Would this be legal, or would it also be

I'm trying to convert this C code to C#, is there a C# equivalent to the C union typedef? struct sockaddr_in { short sin_family; u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; }; typedef struct in_addr { union { struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b; struct { u_short s_w1,s_w2; } S_un_w; u_long S_addr; } S_un; } IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR; Thanks. Darin Dimitrov You may check out the following page . This being said, in .NET you have classes that allows you to work directly with sockets and TCP/IP such as Socket, TcpListener, TcpClient and you don't need to

For a union, writing to one member and reading from other member (except for char array) is UB. //snippet 1(testing for endianess): union { int i; char c[sizeof(int)]; } x; x.i = 1; // writing to i if(x.c[0] == 1) // reading from c[0] { printf("little-endian\n"); } else { printf("big-endian\n"); } //snippet 2(swap bytes using union): int swapbytes() { union // assuming 32bit, sizeof(int)==4 { int i; char c[sizeof(int)]; } x; x.i = 0x12345678; // writing to member i SWAP(x.ch[0],x.ch[3]); // writing to char array elements SWAP(x.ch[1],x.ch[2]); // writing to char array elements return x.i; //

From this question one could start to believe that alignment of a union is not less than the largest alignment of it's individual members. But I have a problem with the long long type in gcc/g++. The full example can be found here , but here are the relevant parts for my question: union ull { long long m; }; struct sll { long long m; }; int main() { #define pr(v) cout << #v ": " << (v) << endl pr(sizeof(long long)); pr(__alignof__(long long)); pr(sizeof(ull)); pr(__alignof__(ull)); pr(sizeof(sll)); pr(__alignof__(sll)); }; This results in the following output: sizeof(long long): 8 __alignof__